Recently, as high performance and miniaturization of the electronic apparatus have been required, high density and high performance semiconductors have been sought. Consequently, circuit substrates for mounting thereof have also been required to be small and of high density. As a result, it is important to design circuit substrates taking the thermal radiation property into consideration. A well known technique for improving the thermal radiation property of circuit substrates, while using a printed circuit board made of glass-epoxy resin, is to use, a metal base substrate having a metal, for example, aluminum etc. and form a circuit pattern on one face or both faces of this metal substrate with an insulating layer interposed in between the circuit pattern and the metal substrate. Moreover, when higher thermal conductivity is required, the metal base substrate is made of a copper plate, which is directly bonded to a ceramic substrate made of, for example, alumina or aluminum nitride. For an application requiring relatively small electric power, a metal base substrate is generally used. In this case, however, in order to improve the thermal conduction, the insulating layer must be thin. Therefore, as for the substrate of thin insulating layer, break down voltage is low, and the influence by the noise, too, is big.
It is difficult for the metal base substrate and ceramic substrate to satisfy both performance and cost requirements. Recently, an injection molded thermally conductive module has been suggested, where a thermoplastic resin composition containing inorganic filler is integrated with the lead frame of an electrode. This injection molded thermally conductive module has excellent mechanical strength in comparion with a ceramic substrate. However, due to the high viscosity of the thermoplastic resin, it is difficult to injection mold such a module with a high filler content, and so the thermal radiation property of module is poor. In particular, at the time of melting the thermoplastic resin at high temperature and kneading with filler, if there is too much filler, the melting viscosity is rapidly increased in a point that not only kneading but also injection molding is made impossible. Moreover, the filler serves as an abrasives to abrade the metallic mold, and, thus, reduces the life of the mold. Consequently, the content of the filler is limited, so that only lower thermal conductivity can be obtained as compared with the thermal conductivity of the ceramic substrate.